8o6 THE POPULAR SCIENCE MONTHLY. 



equilibrium of the blood-supply, and so an inflammatory condition is 

 set up. When this exists the blood-vessels are enormously distended ; 

 consequently, an excess of blood passes through the part, the little 

 cells which secrete the mucus being thus excited and working much 

 more rapidly than when in health. In this way the enonnous discharge 

 of mucus, which accompanies a cold^in the head, is accounted for. 



Another effect of this irritation of the mucous membrane is sneez- 

 ing, which is an effort of Nature to restore the equilibrium of the ner- 

 vous center by another kind of reflex action. Sneezing in catarrh is a 

 method Nature adopts to stimulate the prostrate nervous center, and 

 thus enable it to reassert its proper control over the blood-supply to 

 the part ; indeed, it will be found that the effects of being exposed to 

 a draught of cold air are often comj^letely destroyed by a succession 

 of sneezes. Of course. Nature does not always immediately succeed in 

 these efforts ; but, when she does not, the shock from which the ner- 

 vous center suffers gradually passes away, and the blood-vessels again 

 come under the control of the little nerves which regulate their caliber, 

 and so the catarrh disappears in a few hours, or at most in a few days. 

 It sometimes happens that the shock from the cold air acting upon the 

 nervous center is of such severity that the consequent inflammation 

 is intense enough to check the secretion of mucus altogether, and in 

 consequence the mucous membrane is dry as well as inflamed, and the 

 suffering very much intensified. 



So far, we have only glanced at a cold in the head which j^asses 

 away in a few hours, but this is not always the happy termination. 

 There is a peculiar tendency which inflammation possesses of not leav- 

 ing off where it commenced, but of invading the tissues in its imme- 

 diate neighborhood, and more especially when the tissue is continuous 

 with that primarily attacked, as is the case with the mucous mem- 

 brane of the air-passages. A cold may commence in the head, and 

 rapidly spread by what is technically termed continuity of tissue into 

 the chest ; and so what at the first promised to be only cold in the 

 head may terminate in an attack of bronchitis, or even inflammation 

 of the lungs. Chambers's Journal. 



THE PUPJFICATION OF SEWER -WATERS.* 



Bt M. E. AUBREY-VITET. 



THE sewers of Paris discharge 262,646 cubic metres of liquid mat- 

 ter every twenty-four hours. It is estimated that the quantity 

 discharged will be increased before many years to 300,000 cubic 

 metres daily. Each cubic metre of liquid contains two and a half 

 * Translated and abridged from the " Revue des Deux Mondes." 



